Ultrafast dynamics of Cp*M(CO)(2) (M=Ir, Rh) in solution: The origin of the low quantum yields for C-H bond activation

Using picosecond transient absorption spectroscopy we have examined the dynamics of Cp*Ir(CO)(2)(Cp* = C(5)Me(5)) in cyclohexane and Cp*Rh(CO)(2) in n-pentane solution at room temperature following 295-nm UV excitation. A transient absorption with an instrument limited risetime was observed for Ir from 440 to 740 nm and for Rh from 500 to 650 nm. Each transient can be well fit to a biexponential decay consisting of a fast component of 2-3 ps and a slower component of 30-40 ps. These transients are attributed to excited state absorptions. Taking into account independent femtosecond IR studies of the ground state recovery and our data, we suggest that most excited state molecules relax through nondissociative excited states, decaying to the ground state without the loss of CO. These results offer an explanation for the low C-H bond activation quantum yields observed on preparative irradiation of Cp*Ir(CO)(2) and Cp*Rh(CO)(2).